Bug? cube + mysql.conf

[kao]

1. mysql
config my.cnf
--cut--
innodb_log_file_size = 5M
--cut--
start mysql

2.install roundcube
--
OK
--
start apache && roundcube

OK

change my.cnf
--cut--
innodb_log_file_size = 10M   ..3..40..200...

[1..4] 5 [6 +....]
----------
mysql restart
apache restart
OK
-------------
http://ip/roundcube

##################################
SERVICE CURRENTLY NOT AVAILABLE!

Error No. [500]
------------
###################################

logfile
####################
[Thu Jul 22 11:35:53 2010] [error] [client IP] MDB2 Error: unknown error (-1): _doQuery: [Error message: Could not execute statement]\n[Last executed query: PREPARE mdb2_statement_mysql_1705ce89c49c18509d143bbc9451630e292ac8ef8 FROM 'SELECT vars, ip, UNIX_TIMESTAMP(changed) AS changed\\n     FROM session\\n     WHERE  sess_id=?']\n[Native code: 1286]\n[Native message: Unknown table engine 'InnoDB']\n, referer: http://10.0.1.100/
#####################

changed to
innodb_log_file_size = 5M
works again
Why?
mysql version  5.1
roundcube version 0.3.1